RE: [CAD_CAM_EDM_DRO] Safety -- servo vs. stepper

Servo run away issues are always of concern, but I wouldn't
let them prevent me from using a servo's performance advantages.

I do think about this though... I am pushing around 800+ pounds
at high speed!

My primary machine protection is a hardwired e-stop at ends of travel
followed by a shock absorber tuned to dissipate the kinetic energy
in a specific distance. The e-stop also shorts the servos with a ten
ohm resistor to aid braking.

As I think about it runaways would be caused by two basic scenarios:

1) the computer/interface fails or locks up

2) A servo amp output stage fails such that most of the supply
voltage is applied to the motor.

A watchdog of some kind might be the best way to deal with (1).
I do not know if my emc control is set up for this (I think not). To work
the computer would have to send a signal periodically to the hardwired
estop system to keep it enabled. There is a estop_write bit on emc, but it
is a continuous level and could easily fail in an enable mode.

So what is needed here is a real time function that priodcally spits
out a pulse THRU the interface card (so it's function is also checked).
This might be pretty easy to do perhaps using an extra axis DAC.


Now for (2) (servo amp short) something might work that continually compares
the output current to the input voltage , at least for DC types. If it fails
to be a reasonable facimile of a voltage controlled current source hard
wired
e-stop could be actuated. This also might be pretty easy to do.

A second at least partial safety that I use now is a software velocity limit
a bit lower than the system can do at full tilt. If the velocity (checked by
monitoring a separate tach) exceeds the soft limit e-stop is actuated.


These are just some ideas to improve the safety and robustness of a servo
system.
I would like to hear more ideas from others.

Les

Leslie M.Watts
L M Watts Furniture
Tiger Georgia USA
(706) 212-0242
http://www.lmwatts.com
Engineering page:
http://www.lmwatts.com/shop.html
CNC surplus for sale:
http://www.lmwatts.com/forsale.html
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http://www.lmwatts.com/signwp.html




-----Original Message-----
From: Carl Mikkelsen [mailto:c.mikkelsen@...]
Sent: Thursday, May 20, 2004 12:32 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Safety -- servo vs. stepper


I have a choice of using stepper drive or servo drive for my next
hexapod. My concern is how to cope with safety issues.

My previous hexapod is stepper driven, with fairly slow maximum
speed of just less than 1 inch per second. My next hexapod will be
faster, with a designed intention of fast movements of about 3-5
inches per second.

With my hexapod design, it is very difficult to have limit switches
on all axes. In software, it is easy to limit motions in various
ways, such as to prevent any joint angles or actuator lengths from
being exceeded. It is not easy to place a fixed limit switch on an
actuator to serve as an E-stop of last resort.

Under hardware fault conditions, it seems that steppers are safer.
In particular, most failures in the high-power drive system will not
result in actuator motion. With a servo system, it seems that an H-
bridge failure is likely to result in uncontrolled movement.

Has anyone experience with drive failures, and the reprocussions? I
was about to order servomotors, and my stomach went cold as I
imagined the danger of an sudden, RAPID, uncontrolled movement of a
50 pound mass moving in 3D.

Any thoughts or suggestions?

-- Carl